Why This Works
The radler was created for German cyclists who wanted refreshment without losing the afternoon, which makes the alcohol-free version almost redundant: it was always about the citrus and the fizz.
Grapefruit's bitterness echoes the hops, so the drink tastes more complex than its two ingredients suggest. A pinch of salt sharpens it further if you like.
Use a dry, quality grapefruit soda. An overly sweet one buries the beer; a bittersweet one lets it show through.
The Alcohol-Free Swap
A crisp non-alcoholic lager is ideal. The cleaner and drier the beer, the more the grapefruit reads as bittersweet rather than sugary.